Emission standards around the world continue to place stricter limits on emissions from diesel engines, particularly those in over-the-road vehicles, such as trucks. Additionally, thermal management is increasing in importance as greenhouse gas regulations become more stringent. In order to provide consumers with engines that comply with these standards, manufacturers often employ exhaust after treatment systems that are configured to capture pollutants and/or convert pollutants into acceptable emission constituents.
One type of pollutant that after treatment systems may seek to eliminate and/or reduce the quantity present in exhaust gas is nitrogen oxides (NOx). Systems aimed at eliminating and/or reducing the quantity of NOx may include selective catalytic reduction (SCR) systems. SCR systems typically include an injector that injects a diesel exhaust fluid (DEF) into the exhaust gas. The DEF may be a solution that includes a dissolved reagent, such as, for example, ammonia or urea. The DEF and exhaust gas mixture may then pass through a catalyst in an SCR converter, which reduces NO concentration in the presence of ammonia from the DEF.
The efficiency of the SCR system may be relatively sensitive to the temperature of the incoming exhaust gas. For example, the efficiency of the SCR system may particularly decrease when the temperature of the exhaust temperature entering the SCR system is below a threshold condition, such as below 250° Celsius, and more specifically when the exhaust gas entering the SCR system is around 150° to 250° Celsius. However, during certain engine operating conditions or periods, such as, for example, when the engine is operating under light loads, at low idle, and/or following a cold start, the temperature of the exhaust gas entering into the SCR system may be below a threshold condition for peak SCR efficiency, thereby reducing the ability of the SCR system to convert NOx.
The temperature of the exhaust gas entering into the SCR system may be increased by dosing the exhaust gas, either in-cylinder or in-exhaust gas, with a fuel and combusting the dosed fuel over a catalyst(s) positioned upstream of the SCR system, such as, for example, a diesel oxidation catalyst (DOC) and/or a catalyst(s) in a diesel particulate filter (DPF). However, a relatively significant amount of energy is required to increase the core temperature of these catalysts, which may increase fuel consumption. Further, during light engine loads, such as, for example, during low engine idle conditions, the energy stored in the DOC and DPF may dissipate into the exhaust gas, thus requiring additional energy from the exhaust gas to increase the temperature of the exhaust gas once the engine operating under a higher load.